Interpretive Summary: Hydrologic and water quality models are increasingly used to evaluate the impacts of climate, land use, and land management practices on quantity and quality of land and water resources. Before employing these models for research and/or real-world applications, sources of modeling uncertainty in these models should be identified, minimized, quantified, and reported in order to increase trustworthiness of simulation results. This paper discusses the need for standard calibration and validation guidelines for hydrologic models and presents a framework for the development of the standard by members of the American Society of Biological and Agricultural Engineers. The proposed phases in this important project include, determining the need for the standard, information gathering to build consensus, synthesis of gathered information, and to present a draft of the guidelines based on the synthesis recommendations. The manuscripts submitted to the special collection are introduced and the framework for the synthesis and development are provided. Once developed, these guidelines will provide simple but very important recommendations to users to 1) ensure correct implementation of calibration and validation concepts to minimize uncertainty in model scenario results and 2) provide common documentation and reporting scheme for disseminating results from hydrologic and water quality modeling studies.

Technical Abstract:
This paper introduces a special collection of 22 research articles that present and discuss calibration and validation concepts in detail for hydrologic and water quality models by their developers and presents a broad framework for developing the American Society of Agricultural and Biological Engineers (ASABE) model calibration and validation guidelines. The main objectives of this paper are to: 1) present and discuss the need for standard model calibration and validation guidelines for hydrologic models; 2) introduce and summarize key aspects of the hydrologic and water quality models presented in this collection; and 3) provide the framework for the planned ASABE guidelines for calibration and validation. Advantages for common model calibration and validation guidelines are presented in order to show why they are needed. The models covered in this selection range from field to watershed scales for simulating hydrology, sediment, nutrients and pesticides at temporal scales varying from hourly to annually. Finally, a framework for developing the ASABE model calibration and validation guidelines (CAL-VAL-GUIDELINEs) is presented. The ultimate goal of the CAL-VAL-GUIDELINEs is to provide simple but very important recommendations to users to 1) ensure correct implementation of calibration and validation concepts to minimize uncertainty in model scenario results, and 2) provide a common documentation and reporting scheme for disseminating results from hydrologic and water quality modeling studies.